Internationally seen, iron deficiency is one of the most frequent deficiency diseases which occurs in most developing countries and is also the main deficiency disease in industrial countries. Above all, iron deficiency condition is a problem to women of fertile age but is also to be found in children and young people.
Iron fortification of certain food products is one way to prevent the occurrence of iron deficiency. Since grain products are an important source of consumer goods and are relatively cheap, they are usually chosen as carriers of iron additives.
A suitable iron-fortifying agent must satisfy a number of requirements. First, it must be innocuous to the human body. Furthermore, it must be water-insoluble in neutral or moderately acid environment, which is decisive of good storage properties. It must further have high absorbability in the human body, i.e. good bioavailability, which means good solubility in the gastrointestinal tract at a pH value of about 1 (corresponding to 0.1M HC1). It must also be chemically definable and producible in a reproducible way, i.e. it must have guaranteed constant and controllable properties.
Examples of substances used as iron-fortifying agents are iron powder (reduced iron), iron (III)phosphate iron (III) diphosphate, iron (III) sodium disphosphate, iron(II)sulphate and soluble organic iron(II)compounds. Soluble iron(II)compounds (e.g. iron(II)sulphate) have high bioavailability in the human body. Being easily soluble, they suffer, however, from the disadvantages of causing discolouration and changes in taste of the fortified products when they react with other components in the food, whereby coloured complex compounds and oxidation products are formed which turn the food product rancid. Iron sources which are water-insoluble (e.g. iron powder, iron(III)phosphate) are relatively inert and do not affect the food product to any appreciable extent. The lower solubility in water of these substances is, however, usually associated with low solubility at low pH and thus gives lower bioavailability as compared with soluble iron salts. To date, it has not been possible to find an iron-fortifying agent which in a favourable way combines the requirements for low solubility in water and solubility in the gastrointestinal tract (i.e. at low pH), respectively.
For iron fortification of flour and flour products, reference is made to, for example, U.S. Pat. No. 3,803,292 which describes iron fortification of flour by using a combined iron(II)sulphate preparation. It appears that one has managed to combine the high bioavailability of iron (II)sulphate with long storage life which has been obtained by the use of particles of iron(II)sulphate monohydrate having a surface coating of iron(II)sulphate heptahydrate. However, no bioavailability tests have been accounted for, only stability tests.
Furthermore, Canadian patent specification No. 1,044,945 describes an iron-fortified fruit porridge powder for infants, in which the fortification iron is electrolytic iron (iron powder having a small particle size). No bioavailability tests have been accounted for.
Reference is also made to U.S. Pat. No. 3,876,813 which describes the bioavailability advantage of iron(III)polyphosphate as compared with iron(II)sulphate. These results are, however, not relevant to use on humans since they are based on animal (rat) experiments only. Iron absorption in animals, such as for example rats, is about 100 times higher than in humans, calculated per unit of weight, and furthermore there are considerable distinctions with regard to the capability of absorbing different iron compounds. Thus, studies of bioavailability in humans are required in order to reach a relevant assessment.
The "Handbook of Food Additives", 2 Ed., T. E. Furia, Cleveland, Ohio (1972), p. 660, states that, inter alia, iron(III)orthophosphate and sodium iron pyrophosphate do not, in connection with the fortification of food products, cause the food product to turn rancid, but that their value as fortifying agents is questionable in respect to the bioavailability of iron.
Moreover, a paper by S. Rao and N. Rao, Nutrition Reports International, Vol. 29, No. 5, 1984, pp. 1101-1106, describes the fortification of food products by sodium tripolyphosphate which by chelate bonding with iron can increase iron absorption in the human body.
A comparative study of bioavailability in humans has been carried out with regard to the commercially most frequent iron-fortifying agents which comprise iron(II)sulphate, iron powder and iron(III)phosphate, see Cook et al, "Absorption of fortification iron in bread" in the American Journal of Clinical Nutrition 26, pp. 861-872, August 1973, according to which iron(II)sulphate is considered to have the highest absorbability (p. 864, col. 2, first complete par.). For these comparisons, an isotope measuring technique was used for measuring the absorption of the thus marked fortification iron in the human body, a technique subsequently described in greater detail by Hallberg, "Food Iron Absorption", Methods in Hematology, 1980, pp. 116-133, and by Hallberg et al., "Low bioavailability of carbonyl iron in man: studies on iron fortification of wheat flour", the American Journal of Clinical Nutrition 43, pp. 59-67, January 1986. This technique will be described in greater detail in the Examples below.